This type of ceramic article is glazed either manually or by appropriate robotized machines, after the piece has been formed and dried.
In the case of certain ceramic articles, such as sanitary appliances, the automatic glazing machines are unable to completely glaze all the surfaces of the article, it being possible to glaze only the accessible surfaces, i.e. the visible surfaces.
For example in the case of water closet pans, the rim and the stench trap are currently glazed only on their outer surfaces; likewise it is not possible to glaze the overflow channels of wash-basins and bidets.
Consequently the final product presents unglazed surfaces where, in the course of time, bacterial growth can occur rendering the article unhygienic. Current glazing techniques involve a separate glazing operation which negatively affects the production time and hence cost.
Finally, known glazing machines are costly and complicated.
The object of this invention is to overcome the aforementioned drawbacks of the known art within the context of a rational and reliable solution.
The invention attains said object by virtue of the characteristics defined in the claims.
The invention consists of a method which enables the usual glazing machines to be completely eliminated by glazing the article directly within the forming cavities of the article mould, by injecting into them a glaze compound in liquid suspension, preferably aqueous.
In detail, the method of the invention comprises the following operative steps:    preparing a glaze compound in liquid suspension,    feeding the glaze compound in suspension into the article-forming cavities, for a time sufficient to create a glaze film able to uniformly cover the cavity walls by absorption of the suspension liquid phase;    feeding the slip into the article-forming cavities for a time sufficient to create the article wall by absorption of the slip liquid phase;    possibly feeding the glaze compound into those article-forming cavities in which a film of glaze and a layer of slip have already been deposited, for a time sufficient to create a glaze film able to uniformly cover the inner walls of the layer of slip by absorption of the liquid phase.
The method of the invention can in fact be applied either to the outer and inner surfaces of the article or to only one of the two surfaces, depending on the type of article to be constructed. The method does not require special equipment for its implementation, it using the actual equipment used to form the article.
Said equipment is simply completed by means for feeding and recirculating the liquid glaze suspension, arranged in parallel with the already existing means for feeding and recirculating the slip.
The invention also comprises a glaze suitable for use with the method, which glaze must have characteristics not always present in usual glazes.
A suitable glaze must present, during firing, the same chemical and physical characteristics as a usual glaze, whereas during forming and drying it must behave in a manner similar to the layer deposited by the slip.
The co-existence of said characteristics is obtained, according to the invention, by providing in the glaze composition a clay material component which conveniently exceeds 10% and has the function of increasing the plasticity of the glaze and regularizing its shrinkage.
Said clay material can be an illite, kaolinite, chlorite or montmorillonite clay, or a mixture of said clays. Moreover, said clay material component of said glaze is preferably between 15 and 35%.
Three possible glaze compositions suitable for use in this method are given below in the form of tables.
GLAZE 1GLAZE 2GLAZE 3Raw Materials%%%Sanblend 9020Clay UK120Kaolin LPC20Feldspar F7109K Feldspar FS 9001523.1Quartz P4161813.4CaSiO3 Vansil EW2013.51623ZnO Zinc Oxide181Zircobit MO113(zirconium silicate)Zircosil 513.5(ZrSiO4)Barium Carbonate6.56.55.4Talc SSB 601.51.51.6
If only the outer surface of the article is to be glazed, the glaze suspension in a liquid phase, preferably water, is injected into the mould cavities before forming the article and the liquid phase is then absorbed to create a film of consolidated glaze, of the desired thickness, on the surface of the mould cavity to form the outer surface of the article.
When the film of glaze has been created, the slip is cast in the same mould cavity to form the ceramic article wall, to which said glaze film adheres.
When the article has been removed from the mould, it presents a glazed outer surface.
When the wall of the article has been formed by casting the slip, the inner surface of the article is glazed by injecting the glaze in liquid suspension, preferably aqueous, into the same mould cavity so that a film of consolidated glaze forms adhering to all inner surfaces of the article.
By virtue of the invention, the production of ceramic articles obtained by slip casting is less costly, in that the production plant does not require special glazing machines as glazing is implemented in the actual forming cavities of the mould.
Moreover from the aforegoing, it is apparent that the articles produced using the glazing method of the invention are uniformly glazed in all their parts, both visible and those hidden from the user's view. In certain applications, such as sanitary appliances, it is hygienically very important that the glazing of the article be complete, as this reduces bacterial growth which would render the product unhygienic.
To apply the method of the invention it is sufficient, as described, to provide the mould with two feed circuits, one dedicated to the slip and the other dedicated to the liquid glaze suspension.